This invention relates to a method of making modified abrasive compacts.
Cutting tool components utilising diamond compacts, also known as PCD, and cubic boron nitride compacts, also known as PCBN, are extensively used in drilling, milling, cutting and other such abrasive applications. The tool component will generally comprise a layer of PCD or PCBN bonded to a support, generally a cemented carbide support. The PCD or PCBN layer may present a sharp cutting edge or point or a cutting or abrasive surface.
Diamond abrasive compacts comprise a mass of diamond particles containing a substantial amount of direct diamond-to-diamond bonding. Polycrystalline diamond will typically have a second phase containing a diamond catalyst/solvent such as cobalt, nickel, iron or an alloy containing one or more such metals. cBN compacts will generally also contain a bonding phase which is typically a cBN catalyst or contain such a catalyst. Examples of suitable bonding phases for cBN are aluminium, alkali metals, cobalt, nickel, tungsten and the like.
In use, such a cutting tool insert is subjected to heavy loads and high temperatures at various stages of its life. In the early stages, when the sharp cutting edge of the insert contacts the subterranean formation or workpiece, the cutting tool is subjected to large contact pressures. This results in the possibility of a number of fracture processes such as fatigue cracking being initiated.
As the cutting edge of the insert wears, the contact pressure decreases and is generally too low to cause high energy failures. However, this pressure can still propagate cracks initiated under high contact pressures and can eventually result in spalling-type failures.
In optimising cutter performance increased wear resistance (in order to achieve better cutter life) is typically achieved by manipulating variables such as average abrasive grain size, overall catalyst/solvent content, abrasive density and the like. Typically, however, as a PCD or PCBN material is made more wear resistant it becomes more brittle or prone to fracture. PCD or PCBN elements designed for improved wear performance will therefore tend to have poor impact strength or reduced resistance to spalling. This trade-off between the properties of impact resistance and wear resistance makes designing optimised structures, particularly for demanding applications, inherently self-limiting.
If the chipping behaviours of more wear resistant PCD or PCBN can be eliminated or controlled, then the potentially improved performance of these types of cutters can be more fully realised.
It is known that removing all the metal infiltrant from a layer of PCD results in substantially improved resistance to thermal degradation at high temperatures, as disclosed in U.S. Pat. No. 4,224,380 and GB 1 598 837. JP 59219500 claims an improvement in the performance of PCD sintered materials after a chemical treatment of the working surface. This treatment dissolves and removes the catalyst/solvent matrix in an area immediately adjacent to the working surface. The invention is claimed to increase the thermal resistance of the PCD material in the region where the matrix has been removed without compromising the strength of the sintered diamond.
U.S. Pat. Nos. 6,544,308 and 6,562,462 describe the manufacture and behaviour of cutters that are said to have improved wear resistance without loss of impact strength. The PCD cutting element is characterised inter alia by a region adjacent the cutting surface which is substantially free of catalysing material. This partial removal (up to 70% of the diamond table being free of catalysing material) is said to be beneficial in terms of thermal stability.
Methods for the removal of the catalysing material that are mentioned in these patents are acid etching processes (for example, using hot hydrofluoric/nitric acid or hydrochloric/nitric acid mixtures), or electrical discharge or other electrical or galvanic processes, or thermal evaporation. These methods, however, do not take into account the variation in the composition of the metal matrix. Sintering of abrasive compacts is carried out in high temperature-high pressure presses that have a degree of variability in the pressure and temperature conditions that they produce. This variability is exacerbated by the difficulty of monitoring the high pressures and high temperatures required for synthesis and sintering.
The process variability is caused by gradual ageing of press components with use, by variations in the physical dimensions and properties of the capsule components, and by pressure and temperature gradients within the capsule. These gradients can be minimised by careful choice of the materials of construction of the capsule components and by the overall design of the capsule. Furthermore, the pressure-temperature-time operating conditions for the press can be developed to minimise such gradients. However, the gradients can never be totally removed.
A much larger and unavoidable source of variability is the different process conditions required to sinter different PCD or PCBN products, which by design have different grain sizes, different layer thicknesses, different layer compositions and different overall heights and outer diameters.
All of the abovementioned sources of variability result in differences in the final composition of the metal matrix. The variability in the composition of the metal matrix results in variable rates of removal of the metal matrix, as certain components of the metal matrix will be more susceptible to the method of removal, and some will be less susceptible. Where the source of variability in the metal matrix composition is within a capsule, this results in variations in thickness of the thermally stable layer within an abrasive compact, and this is unacceptable, as it translates into areas of better and poorer performance on an abrasive compact.
Where the source of variability is the press or the press conditions, in other words external to the capsule, it necessitates the continual adjustment of the conditions under which the catalysing material is removed according to the specific abrasive compact product. From a production point of view, this is inconvenient and potentially more costly.